1. Field of the Invention
This invention relates to a silica-alumina catalyst support having a median pore radius ranging from about 65 to about 130 Angstroms and having a total pore volume ranging from about 0.75 to about 1.3 ml/g. The support is particularly useful as a base for hydrotreating catalysts in the desulfurization of heavy petroleum feedstocks.
2. Description of the Prior Art
Heavy hydrocarbon feedstocks such as crude oil, atmospheric distillation column bottoms or reduced crude oil, and vacuum residuum, often have a high sulfur and high metals content. The metals normally comprise nickel, vanadium and iron, and the sulfur is normally in the form of sulfides and/or mercaptans. Removal of these impurities is usually required to prodce clean transportation or heating fuels and to facilitate various downstream refinery processes.
Desulfurization and demetallation are advantageously accomplished by passage of the hydrocarbon feedstock over a catalyst in the presence of hydrogen. Commonly used catalysts for this purpose include Group VI-B and Group VIII metals, their oxides or sulfides, on alumina or silica-alumina supports.
It has long been recognized by those knowledgeable in the art that the catalyst support characteristics such as surface area, pore volume and pore size distribution play a vital role in the activity and stability of the selected catalyst. If the support pore volume is comprised primarily of small-diameter pores, such as pores having a median radius of approximately 35 Angstroms, the catalyst tends to foul more rapidly than catalysts having larger pores, and the rate of reaction is diminished due to the slow diffusion rate of the impurity-bearing hydrocarbon molecules through the smaller diameter pores. Thus, attention has focused upon the preparation and discovery of suitable support materials having the desired pore characteristics.
The following references disclose certain catalysts and/or supports which were formulated in attempts to overcome the deficiency of the prior art catalysts:
U.S. Pat. No. 3,876,523 discloses hydrodemetallation and hydrodesulfurization alumina-based catalysts having an average pore diameter between 150 to 250 Angstroms.
U.S. Pat. No. 3,891,514 discloses a demetallation and desulfurization catalyst comprising a hydrogenation component composited with an alumina support, the pores of which are distributed over a range of 180 to 300 Angstroms in diameter and having a surface area in the range of 40 to 70 m.sup.2 /g.
U.S. Pat. No. 3,393,148 discloses a hydroprocessing catalyst comprising a hydrogenation component in a large-pore-diameter alumina having a surface area ranging from 150-500 m.sup.2 /g and an average pore diameter ranging from 100 to 200 Angstroms.
U.S. Pat. No. 3,471,399 discloses a hydrodesulfurization catalyst comprising silica-alumina and a hydrogenation component, said catalyst having an average pore diameter ranging from 70 to 90 Angstroms and a surface area of about 150-250 m.sup.2 /g.
U.S. Pat. No. 3,322,666 discloses a hydrodesulfurization catalyst comprising a hydrogenation component on an alumina support which has the majority of its pores in the 15-50 Angstrom radius range.
U.S. Pat. No. 4,066,574 discloses a hydrodesulfurization catalyst having an alumina support, which support has at least 70 volume percent of its pore volume in pores having a diameter between 80 and 150 Angstroms and less than 3 volume percent of its pore volume in pores having a diameter above 1000 Angstroms.
None of the above patents, however, teach the use or preparation of a silica-alumina support having the particular pore size distribution disclosed herein.